As if wine grape growers don’t have enough challenges in this day and age, the effects of grapevine viruses have been taking on greater importance in eastern vineyards over the past several years. Studies examining grapevine leafroll-associated viruses are developing a growing body of information that will be essential for vineyard managers to continue moving the eastern wine grape industry forward. Grape growers in the eastern United States need not feel they are the only ones with this disease management challenge (as is the case with many fungal diseases of grapes); grapevine leafroll-associated viruses (GLRaVs) are found in vineyards all over the world (Compendium of Grape Diseases). This group of viruses causes a disease known as grapevine leafroll disease, and the association of symptoms with grapevine leafroll viruses was recognized over 80 years ago. As is the case with so many plant pathogens, the worldwide distribution of these viruses occurred as a result of increased movement of plant material/goods across the globe; the ever widening dissemination of infected planting stock (Compendium of grape diseases). The effects of these leafroll viruses is most severe on – you guessed it – cultivars of V. vinifera, where the disease is known to greatly reduce yield, vine vegetative growth or vigor, and cold hardiness; a factor of critical importance for these cultivars grown in the northeastern United States. Grapevine leafroll disease can also delay fruit maturity, reduce color development in red grapes, and fruit quality (decreased soluble solids, increased titratable acidity) of V. vinifera grapes (Fuchs et al. 2009), which can negatively impact perceived wine quality. The severity of the effects of leafroll viruses is dependent on a great number of factors such as grapevine cultivar, virus strain, climate, soil, cultural practices, stress factors, etc. So naturally, the severity of symptoms can vary from one season to the next (Compendium of Grape Diseases). With respect to cultivar, the effects of these viruses on Vitis interspecific hybrids and Vitis labrusca are generally considered to be less serious, but are also less well defined and studied.

Infection by leafroll viruses results in the degeneration of primary phloem tissues in grapevine shoots, leaves and clusters (Compendium of Grape Diseases). As one can imagine, this can have profound effects on all parts of the vine. Symptoms of the disease, which are generally most observable on V. vinifera, consist of cupping and discoloration of older leaves in late summer and fall. On red fruited varieties, leaves of infected vines can display a distinct red coloration of the interveinal tissue, while veins remain green (Figure 1). On white fruited varieties of V. vinifera, symptoms are less striking and leaves tend to look yellowish (chlorotic) and cupped (Figure 2). Leaf discoloration generally affects older leaves first, but these symptoms are not diagnostic of the disease, as they may be due to other causes such as nutrient deficiencies, water stress, and even crown gall. Analysis of grapevine tissues in the laboratory is the only way to confirm the presence (or absence) of these viruses.

Currently, there are about seven GLRaVs found in cultivated grapes, the most common being GLRaV-3. These viruses are easily spread over long distances through the movement of infected nursery stock, but can be spread (vectored) within the vineyard by mealybugs (Compendium of Grape Diseases). Unfortunately, there are no known sources of resistance to GLRaVs among Vitis species and they have been found in many cultivated grape varieties, including V. labrusca, Vitis interspecific hybrids, and V. vinifera. Interest in grapevine leafroll disease and the extent of its effects has been growing in the eastern United States over the past ten years or so. Surveys conducted in New York, Ohio, and Virginia (Fuchs et al. 2009, Jones et al. 2015, Han et al. 2014), have provided confirmation of the presence of GLRaVs in commercial vineyards and have yielded important information necessary to the management of grapevine leafroll disease. For example, infection by GLRaVs is permanent and infected vines must be destroyed to reduce the incidence of grapevine leafroll disease. Therefore, management of the disease would naturally include planting only stock that is free of GLRaVs. Insecticides that target mealybugs and soft scales can prevent vine to vine spread (within the vineyard) of GLRaVs that are known to be vectored by these insects (Compendium of Grape Diseases). Indeed, studies have shown that applications of insecticides like dinotefuran (Scorpion) and spirotetramat (Movento) can significantly reduce mealybug counts and result in a slowing of the progress of the disease in vineyards. One study from New York (Fuchs et al. 2015) showed that insecticide applications should target overwintered and second instar mealybug crawlers from bud swell to bloom and summer generation crawlers later in mid-summer. A study with grape phylloxera as a potential vector of these viruses showed that phylloxera can acquire the virus through phloem feeding on infected vines, but there was no evidence that phylloxera can transmit it (Wistrom et al. 2017).

As was mentioned earlier, cultivars of Vitis labrusca (Concord, Niagara) can also become infected with GLRaVs, but the infections appear to remain latent or dormant (Bahder et al. 2012) and have not been shown to result in visual symptoms of the disease (Wilcox et al. 1998). On the other hand, cultivars of V. vinifera are severely affected by GLRaVs and make up a very important and growing sector of the PA wine grape industry. Surveys conducted in New York, Ohio, and Virginia (Fuchs et al. 2009, Jones et al. 2015, Han et al. 2014) have revealed the presence of GLRaVs in commercial vineyards to the north, west, and south of Pennsylvania and have led to the development of some important guidelines for management of grapevine leafroll disease.

Given the fact that grapevine leafroll disease is common worldwide and that grapevine leafroll disease can profoundly impact wine quality and grapevine health, researchers at Penn State University are initiating a project to look for GLRaVs in Pennsylvania vineyards. As in other states, the study is targeted to help growers recognize the impact that the disease may be having on the Pennsylvania wine industry and help them to address the effects of these viruses on productivity and fruit quality, reduce their spread and impact, and thereby grow and improve the wine grape industry in Pennsylvania.

The short term, initial objectives of this project will focus on the development of an online survey to collect information from growers with regard to the presence of symptoms of grapevine leafroll disease in Pennsylvania vineyards and their interest in participating in the project. The project will then follow up with tissue sampling from participating, symptomatic and non-symptomatic vineyards throughout the state and serological analysis to determine the presence of Grapevine leafroll virus-1 and Grapevine leafroll virus-3 – the most common of the leafroll viruses – in commercial vineyards in Pennsylvania. The collection of vineyard samples across the state will map the incidence and geographical distribution of these viruses on cultivars of Vitis vinifera and Vitis interspecific hybrid grapevines. The project will also determine and compare the impact of grapevine cultivar and age on infection by Grapevine leafroll virus-1 and -3 in Pennsylvania. Once infected vines have been identified in Pennsylvania vineyards, future objectives will focus on studying the impacts of grapevine leafroll disease on grape quality and productivity in Pennsylvania, and management techniques to mitigate the economic impact of the disease on the Pennsylvania wine industry.

Vineyards will be selected from all parts of Pennsylvania, but the number of locations will favor northwestern and southeastern PA, where the majority of vineyards are located. The study will be expanded as new findings are made and the results will be made available to growers at various meetings throughout the next several years.

The Penn State Extension grape team has been receiving reports on herbicide drift damage in vineyards from a number of Pennsylvania wine grape growers this growing season, definitely many more than in previous years. All herbicides registered for grapes can potentially harm the vines if not applied in accordance to the pesticide label (e.g., glyphosate products) [1]. However, in many of the reported cases through the 2017 growing season the damage was caused by herbicides not registered for grapes, which drifted into the vineyards from nearby fields.

Damage from herbicide drift is, unfortunately, something that grape growers across the country are too familiar with. It represents an economic threat for the grape and wine industry and should not be underestimated. Herbicide drift damage can, indeed, result in significant crop losses which may extend to multiple seasons, and in some cases it also results in vine death. Several extension web resources are available to assist grape growers in preventing and dealing with herbicide drift damage. Some of them are listed at the end of this article, including one from Andy Muza, extension educator at Penn State (Growth Regulator Herbicides and Grapes Don’t Mix).

Due to the increase in reports of herbicide drift damage in Pennsylvania vineyards it seems appropriate to discuss some key points surrounding this issue. This article will review how to identify herbicide drift symptoms, what measures grape growers and pesticide applicators can take to prevent herbicide drift, and what steps to take if the drift occurs.

Plant growth regulators (PGR) herbicides are those most likely to injure grapevines, mainly through drift.

I will only focus on the herbicides which belong to the plant growth regulators (PGR) mode of action group. Common active ingredients of PGR herbicides are 2,4-D (2,4-Dichlorophenoxyacetic acid; phenoxy family), dicamba (benzoic acid), tricolopyr or picloram (pyridine family). A partial list of common PGR herbicides as well as other herbicides that may injure grapevines can be found at Preventing Herbicide Drift and Injury to Grapes, Table 1.

PGR herbicides are widely used for controlling broadleaf weeds in many crops, such as wheat, corn, soybean, pasture, rangeland, etc. They are also frequently utilized to control unwanted broadleaf vegetation in turf, by railroads, road ditches, fence lines, and rights-of-way. These herbicides are not registered for use with grapes. However, when applied to a nearby field, they can drift into the vineyard and cause significant injury to grapevines.

Most of the herbicide drift damage reported this season by Pennsylvania grape growers were caused by drift of PGR herbicides (Figure 1). Physiological symptoms to PGR exposure is not too surprising because grapevines are extremely sensitive to PGR herbicides, including the phenoxy, benzoic, and pyridine classes of compounds [2]. For example, herbicides containing 2,4-D can damage grapes at a concentration 100 times lower than the recommended label rate. Moreover, drift from PGR herbicides can injure grapevines located half a mile or more from the application site.

Drift is defined as “the movement of herbicides off the site where they were applied” [3]. Non-target drift can occur either as spray drift or vapor drift. Spray drift occurs during herbicide application when small droplets move off the application site under unfavorable wind conditions. Vapor drift occurs after herbicide application as the spray material volatizes or evaporates and is carried away from the application site by wind or temperature inversions. Some PGR herbicides, such as ester formulations of 2,4-D, readily volatilize, especially when used under high temperatures and low humidity conditions (high vapor pressure) [3].

Damage from PGR herbicides typically appears within 2 days of the drift occurrence. Herbicide drift can damage leaves, shoots, flowers, and fruit. Leaf symptoms are often easy to recognize, but sometimes can be mistaken with those of fanleaf degeneration, a viral disease [3]. Growers can send pictures of damaged vines to a local extension specialist for confirmation.

Typical symptoms include:

Distorted leaf appearance: Symptoms are typically more severe on the youngest leaves and shoot tips. Affected leaves are “smaller, narrow, deformed, and they have closely packed, thick veins that lack of chlorophyll” [4]. They may also have a distinct fan-shape appearance, and depending on the herbicide’s active ingredient, they can bend downward or cup upward (Figures 2, 3). Leaves may or may not outgrow the symptoms, it largely depends on the severity of the injury and other factors listed in the following section (“Factors affecting the severity of injury”). It is also common to see regrowth of deformed leaves after drift exposure [3].

Flower clusters (inflorescences): symptoms can include aborted or failed flowers, and poor fruit set (Figure 4). If the injury is severe enough it can cause reduced yield at harvest and poor fruit quality, in addition to potentially illegal residues of herbicide on the exposed crop.

Figure 4. 2,4-D herbicide drift damage on Grüner Veltliner flower clusters. Photo taken on July 19, 2017 approximately two months after the herbicide drift incident. Notice only two berries developed properly (circled in the photo).

In some cases, depending on the timing and level of drift exposure, floral symptoms may be much more pronounced than those on the leaves making the diagnosis more difficult (i.e., growers may relate poor fruit set or dead flowers to other causes rather than herbicide drift) (Figure 5).

If the damage occurs early in the season, between bud burst and bloom, as it usually does, a significant reduction in healthy leaf area during the period of rapid shoot growth may affect vine photosynthetic capacity, lowering vine ability to fully ripen the crop and possibly its ability to survive cold winters.

Unfortunately there is no guarantee that the vines will fully and rapidly recover from herbicide drift damage. Carry-over effects into the following years, such as reduction in vine vigor, yield, fruit quality, and increased susceptibility to diseases, are common if the damage is extensive and/or the vines have been repeatedly exposed to PGR-herbicide drift. Finally, vines may die as a consequence of their weakened condition [2].

What factors affect the severity of PGR-herbicide drift damage?

Some of the most important factors affecting the severity of drift damage are:

Vine growth stage at the time of exposure. Grapevines are always sensitive to PGR herbicides, but they are most susceptible during the early part of the growing season, from bud burst through bloom. While dependent on the growing season and site, in Pennsylvania this usually occurs around April through June. Early in the growing season shoots are rapidly growing and PGR herbicides are quickly translocated to the shoot tip, where the natural concentration of auxins is greatest inside the grapevine. If exposure occurs later in the season, vines typically outgrow the damage and still produce good yield [5].

Vine age: Younger plants are more vulnerable and they have a lower ability to recover from the PGR herbicide damage than mature vines. Young vines may be killed even at low exposures [6].

Level of exposure: Higher concentration and/or repeated exposures will result in higher disruption of the vine’s physiology and lower ability of the vines to rapidly and fully recover from the damage [3].

Other factors include herbicide concentration and formulation (for example ester formulations of 2,4-D are more volatile than amine formulations, thus ester formulations of 2,4-D are more prone to move off-target as vapor), weather conditions (temperature, humidity, and most importantly wind speed) at the time of herbicide application.

What is the best strategy to protect vines from herbicide drift injury?

Prevention is undoubtedly the best strategy for grapevine growers to avoid herbicide damage. To reduce the risk of herbicide drifting into their vineyard, vineyard managers and/or owners should be proactive. Some prevention steps both grape growers and nearby growers of other crops can take are listed below:

Maintain good relations with neighbors. Vineyard owners and managers should make sure their neighbors within approximately a half-mile to 1 mile radius, are aware that vines are extremely sensitive to PGR herbicides [3]. It is also recommended to encourage neighbors to “use drift-reduction spray nozzles (nozzles that produce large droplets) and to select herbicides that are less likely to injure grapes” [3]. If growers of other crops are unaware of damage to grapevines, collecting information such as this blog post, may be an important educational tool to share. Mike White, viticulture extension specialist at Iowa State University, suggests to share an aerial map of the property showing the vine­yard location with neighbors and commercial pesticide applicators to increase their awareness. It is also recommended to communicate the presence of the vineyard to state and county highway departments.

Windbreak (shrubs, trees, physical barriers) and a buffer area between the vineyard and the edge of the field being sprayed are always a good idea. Penn State offers a free publication or pdf print-out regarding windbreaks: http://extension.psu.edu/publications/uh172/view

For those states where the service is available, growers can register the location of their vineyard on https://driftwatch.org/. This online service is not available in Pennsylvania, but in many Midwestern states growers and pesticide applica­tors can use this web resource free of charge to report (growers) and locate (applicators) potential drift hazards.

Taking all these steps may not guarantee that herbicide drift will not occur in your vineyard, but increasing pesticide applicators awareness of grape sensitivity to PGR herbicides, the resulting economic loss, and potential litigation risks may very well serve the purpose.

Applicators should always follow all the measures available to minimize the risk of herbicide drift into a nearby vineyard or to other sensitive crops. Legal complaints may result in expensive settlements. In an extreme example, an owner of a 150-acre vineyard in Australia was awarded AUS$ 7M in damages over pesticide drift (Grape grower Awarded $7M in damages over spraying) that occurred from 2013 to 2015.

If PGR herbicides are applied after vine bud burst, applicators should consider eliminating volatile compounds and apply only non-volatile products.

Extension personnel could also facilitate communication between grape and crop field growers as it happens in Long Island, NY. Extension personnel from Cornell University-Long Island, including Alice Wise and Andy Senesac, organized a meeting with local grape and sod growers to tackle the herbicide drift issue which was affecting local grape growers without having to resort to regulatory restrictions. The result of that meeting was a ‘gentleman’s agreement’ not to spray herbicides containing 2,4-D after April 15, around bud burst for the earliest grapevine varieties in Long Island. To keep all parties informed, extension sends out a weekly reminder about this issue.

What to do if the drift occur

Here some key steps Mike White put together on what to do right after a drift incident [7]:

Identify area affected.

Document the date, time and growth stage of the grapes.

If possible, identify the source of the drift and make a determination if you want to settle the problem amongst your neighbors.

Contact your state department of agriculture (Pennsylvania Department of Agriculture, PDA) as soon as possible if you cannot determine the source of the drift and/or you want to formalize the complaint (30 – 45 day deadline in many states).

Flag both affected and unaffected plants, take high reso­lution pictures weekly until symptoms subside and measure final yields per plant.

Severe injury settlements should be delayed until after next season’s harvest. Photo and yield documentation should be continued. Unless the settlement offered seems exceptionally lucrative, I would suggest delaying any settlements until after next season’s harvest to assess for potential carry-over vine damage.

There is limited information available on best management practices for vines affected by herbicide drift damage. To favor a full and a rapid recovery it is recommended to still implement good management practices and avoid further stress to damaged vines, as for example over cropping (assuming damaged vines have fruit). Fungicide applications made to protect the fruit should not be necessary if the fruit has been removed [8]. It is also recommended to adjust pruning strategies to smaller vines, with the intent of regaining full vine size [9].

As we move into the post-bloom period, we are reminded that the immediate pre-bloom spray and the first post bloom spray are the most important you’ll make all season. These two sprays protect the nascent crop during its most vulnerable period and are essential to a fruit disease management program for control of the four major grape diseases; powdery and downy mildew, black rot, and Phomopsis. Use ‘best’ materials, shortest intervals, best coverage, etc., for those two sprays, EVERY YEAR! No matter what varieties you grow, those two sprays are the most important for protection of your crop. For growers of Vitis vinifera and many of the French hybrids, the second and perhaps third post-bloom sprays are also of critical importance, especially in a wet year and in vineyards that have already developed some observable level of disease this season. That said, let’s review these major diseases.

First, there’s Black rot caused by the fungus Guignardia bidwellii. This fungus can infect all immature green parts of the vine: fruit, shoots, leaves, and tendrils. On leaves, infections start out as small light green spots visible on the upper surface gradually turning brown to reddish-tan as infected tissue dies (Figure 1). Small, black, pimple-like bodies (pycnidia) develop inside the spot or lesion, usually arranged in a loose ring just inside the dark brown edges of the spot (Figure 1). Spores of the fungus are formed within pycnidia, and are released and splashed around during rainfall periods. Leaves remain susceptible as long as they are expanding and the size of leaf lesions indicate when, during expansion, the leaf was infected. For example, small lesions result when leaves become infected near the end of their expansion. Large lesions indicate the leaf was infected early in expansion. However, numerous small lesions, when clustered, may coalesce to damage large portions of the leaf. The death of large portions of the leaf blade may cause the entire leaf to die and abscise, but this is rare. On petioles, black, elongated lesions may induce wilting or abscission of leaves. Infections on berries initially appear as small, tan spots that develop a dark outer ring and expand rapidly to rot the entire berry. The brown berry shrivels into a hard, black, wrinkled mummy studded with spore producing pycnidia (Figure 2). Once the caps come off during bloom, berries of most varieties are highly susceptible for about 3-4 weeks, gradually developing resistance 5-6 weeks after capfall. Infections that take place during peak susceptibility generally show symptoms within 10-14 days. As berries develop resistance to black rot, the time for infections to become manifest takes longer, and infections that occur toward the end of the susceptibility period (second half of July?) may not develop symptoms until veraison.

Fig. 1 Development of black rot lesions on grapevine leaf (Concord).

Fig. 2 Development of black rot lesions on grape berry (Concord).

On shoots, lesions appear as elongated or elliptical brown cankers. Pycnidia may be clumped in the center of the lesion and/or line the margins of the lesion (Figure 3). These pycnidia produce spores during the current season and can be a source of further infection to fruit. These lesions remain on the shoots after they have “hardened off” and can survive over winter to release spores again the following spring. Large shoot lesions may render the shoots susceptible to breakage by wind, but this is rare.

Fig. 3 Black rot shoot lesions (Concord).

As berries develop resistance, the appearance of new infections may change: circular lesions are black, expand more slowly, and may remain small, often failing to affect the entire berry (Figure 4). Likewise, leaf infections that take place at the very end of the susceptibility/expansion period may become manifest as small dark pinhead size spots that do not expand (Figure 4).

Fig. 4 Limited black rot lesion development from infections occurring toward the end of the susceptibility period (Concord).

Cultural and chemical control:

The black rot pathogen survives the winter in infected grape tissue (primarily fruit mummies) which serves as a source of inoculum (spores) the following season. Inoculum that remains in the trellis poses a much greater risk than inoculum dropped to the ground. Therefore, one of the most important methods of cultural control of black rot is removal of infected material, particularly fruit and cluster material, from the trellis. Once on the ground, mummy viability is reduced to further improve control. To take matters a step further, row middles can be plowed and hilling up under the row can bury mummies directly under vines. Maintaining an open canopy where fruit and other susceptible tissue dry out as quickly as possible after rainfall, will also help reduce this disease and improve fungicide penetration and coverage of the fruit.

Chemical control options for black rot mostly include two modern active ingredient classes like the strobilurins (azoxystrobin, kresoxim-methyl, pyraclostrobin, trifloxystrobin) and the sterol inhibitors (tebuconazole, tetraconazole, difenoconazole, myclobutanil) as well as the old standards like captan, mancozeb, and ziram. All are quite effective. The strobilurins and sterol inhibitors are more rainfast than the old standards and the sterol inhibitors have the capacity to stop the progress of an existing infection if applied within about 3 days after the infection period.

Scouting can be an important part of a black rot control program. The presence of pre-bloom leaf infections, especially those in the fruit zone, may indicate the presence of an over-wintering source of inoculum in the trellis and high risk of fruit infection after capfall. Fruit infections can occur during bloom and anytime up to 5-6 (native varieties) to 7-8 (Vitis vinifera) weeks after bloom.

In most parts of Pennsylvania, downy mildew first became active during the second half of May; at about the 5-6 leaf stage of grapevine development. Up here along the southern shore of Lake Erie, our first infection period occurred on May 25 (rainfall with temperatures above about 52 F) and first symptoms were observed at our farm on unprotected suckers of Chardonnay on June 1 (about 6-7 days after infection). On leaves, the first infections of downy mildew appear as yellowish ‘oil spots’ on the top of the leaf that coincide with a white, fluffy or downy patch of sporulation on the lower surface. On young shoots and clusters, early symptoms may first cause cluster rachises and shoots to thicken and curl (Figure 5). As the pathogen, Plasmopara viticola, aggressively colonizes young, expanding grape tissue, infected shoots, clusters, and leaves may turn brown and die. When berries are infected later in the season their development is hindered and they fail to soften at veraison, turning a pale mottled green (white varieties) to red or pink (red varieties, Figure 6). Inflorescences and fruit clusters are most susceptible from about 2 weeks pre bloom to about 2 weeks post bloom. Highly susceptible varieties will require protection through 3-4 weeks post bloom because cluster stem tissue may remain susceptible until later in the season (after fruit have already become resistant) and cluster stem infections can still result in fruit loss. Young leaves and shoots are very susceptible, but become somewhat more resistant as they mature.

Fig. 5 Infection of downy mildew on young cluster and shoot showing curling and thickening of diseased tissue (Chancellor). The white sporulation after a warm humid night can be striking.

Fig. 6 Berries of red varieties (Concord (left) and Chancellor (center) at harvest) often turn red or pink after infection and fail to soften and develop properly. Late season leaf infections (far right photo) are yellowish to reddish brown and appear angular or blocky.

Cultural and chemical control:

Because the first inoculum arises from the vineyard soil, cultivation in early spring can help to bury over-wintering inoculum in old leaves and clusters on the ground, reducing primary inoculum in spring (much like with black rot). The first infections in spring often occur on shoots and sucker growth near or on the ground, and prompt elimination of this tissue can delay the occurrence of the first infections in the canopy. Also, the maintenance of an open canopy, where fruit and other susceptible tissue dry out as quickly as possible after rainfall and dew, will help minimize disease development.

There are many chemical options for downy mildew control and the best materials should be applied around and shortly after bloom. Active ingredients found in Ridomil, Zampro, Presidio, and Revus (and Revus Top) have been most effective on downy mildew in our trials. Where strobilurins are still working on this disease (no resistance yet), Abound (except in Erie county), Pristine, and Reason have been very effective too. The phosphorus acid formulations like Phostrol, Prophyt, and Rampart to name a few, have also been very effective against downy mildew, but generally cannot be expected to provide good control beyond 10 days after application, especially under high disease pressure. A tank mix of Ranman (cyazofamid) and phosphorus acid has been shown to be very effective on downy mildew in many university trials. All these aforementioned materials are very rainfast. In addition to these fungicides are the old standards that are strictly surface protectants and are more subject to removal by rainfall. A mancozeb product is probably the best among this group, but fixed copper fungicides can also be quite effective against downy mildew on varieties that are not sensitive to copper. Ziram and captan can also be part of an effective downy mildew program, but are somewhat less effective than mancozeb.

Powdery mildew is caused by the fungus Uncinula necator. Infection on leaves appears mainly on the upper surface as white, powdery patches, though the undersides of leaves can also become infected (Figure 7). As the leaf surface becomes covered with the fungus, leaf function (and photosynthesis) is impaired, with varieties of V. vinifera and highly susceptible French hybrids being most severely affected. Infection by U. necator can stunt growth of new tissues and severe infection of young expanding leaves often results in cupping and distortion of leaves. Cluster infections around bloom may lead to poor fruit set, while later infection can cause berry splitting.

Fig. 7 Powdery mildew on young, developing ‘Concord’ berries.

Though primary infections in spring (at least 0.1″ rainfall and greater than 50 F) require rainfall for spore release, secondary disease cycles that result from primary infections, do not require rainfall. Under optimum weather conditions (temperatures in the mid 60s to mid 80s F) secondary disease cycles can be repeated every 5 to 7 days, allowing for explosive increase of disease in the vineyard, especially in highly susceptible wine varieties. Note that optimum temperatures for the fungus are the norm through most of the summer in Pennsylvania and that starting around bloom, nearly every day is an infection period, rain or shine.

In most grape varieties, berries are highly susceptible to infection from the immediate pre-bloom stage until about 2-3 weeks after fruit set, and efforts to protect fruit with fungicides should concentrate on this critical period with timely applications every 7-14 days. Cluster rachises and leaves remain susceptible until harvest and their need for continued protection depends on varietal susceptibility, crop size, and weather. For example, after the fruit susceptibility period, further management of leaf and rachis infections may not be necessary on Concord and other native juice varieties unless vines are heavily cropped or ripening conditions are poor. On the other hand, V. vinifera and susceptible hybrids, may require management of foliar mildew until at least veraison or beyond.

Cultural and chemical control:

There are cultural considerations that can reduce opportunities for powdery mildew disease development. Most involve limiting humidity and promoting sun exposure to all parts of the vine. For example, a training system that improves air movement through the canopy, prevents excess shading and humidity and promotes fungicide penetration to the cluster zone which will help reduce powdery mildew development. Sunlight is lethal to powdery mildew and regular exposure of leaves and fruit can greatly reduce mildew development. Good weed control can also minimize humidity levels that contribute to mildew development.

Unfortunately, cultural measures can only serve as an enhancement to a chemical control program in Pennsylvania and other parts of the northeast. However, we have many effective fungicides for powdery mildew that can provide high levels of control through the critical period around bloom: Vivando, Quintec, Luna Experience, Endura, and now Aprovia. Aprovia is also labeled for black rot control, but our recent tests have indicated that Aprovia’s black rot efficacy is limited especially under high disease pressure on susceptible varieties. The difenoconazole products (Revus Top, Quadris Top, Inspire Super) can also be very effective on powdery mildew, though they may best be used outside the critical two spray period around bloom. Be aware that difenoconazole has been found to cause injury to Concord and a few other varieties (read the label). Sulfur can be an effective powdery mildew material too (on sulfur tolerant varieties) and many wine grape growers rely heavily on it, especially as a tank mix pre-bloom with mancozeb for all diseases. However, it is not recommended as a ‘stand-alone’ material during the critical fruit protection period for powdery mildew control.

There are lots of ‘alternatives’ for powdery mildew control that may be appropriate for late season sprays (to maintain a clean vineyard) that may gradually be used to replace the sulfur and/or synthetics or rotate with synthetics, particularly for reds where late sulfur applications can create wine quality issues. These are materials for which there is little risk of the development of resistance. In fact, these materials can be used to manage the development of resistance to our more risky synthetic fungicides mentioned earlier. Petroleum based oils like JMS Stylet-oil are very effective at 1-2 % solution, but excessive use late in the season (do not apply around or after veraison) may limit sugar accumulation and fruit maturity. And, oils should not be tank mixed with sulfur or applied within 14 days of a sulfur-containing fungicide application. Copper, is moderately effective on powdery mildew and generally applied with lime to reduce the risk of phytotoxicity (read the label). Like sulfur, copper fungicides should not be applied under slow drying conditions as this increases the chance for plant injury. Other materials include potassium bicarbonates such as Kaligreen, Armicarb O, and Milstop. These materials generally produce modest results, and are most effectively applied at short intervals (7 days) to achieve satisfactory control on susceptible varieties. Again, these materials are not appropriate for the critical fruit protection period, but are best integrated during the early season when disease pressure is low OR after the critical fruit protection period to help control leaf infections.

Phomopsis cane and leaf spot is caused by the fungus, Phomopsis viticola. Earlier this spring, growers in many parts of Pennsylvania experienced problems with Phomopsis development on new shoots and leaves. Prolonged wetting/rainfall during the first week of May led to widespread infection by this pathogen on Concord in the Lake Erie region; virtually every shoot of every vine in every Concord vineyard we have examined has some level of Phomopsis development on the first one or two internodes. The infection period(s) occurred when shoots were in the 1-3″ range and inflorescences were just becoming exposed. In some cases, heavy infection of inflorescences is likely to result in problems with fruit rot after veraison (months after the infection period took place!). Fruit are generally at risk of new infections until a couple weeks or so after bloom, but infections of the cluster stem tissue that occur in the early pre-bloom period can move into berries during ripening and cause fruit to rot and shell before harvest. The concentration of heavy infection at the base of the oldest internodes, may result in large scabby areas that weaken the shoot (Figure 8) and green shoots that are severely infected are more apt to break under windy conditions. Leaf infections appear as pinhead sized black spots surrounded by a yellow halo (Figure 9). These infections appear to be of little consequence, other than revealing the presence of the pathogen. Lesions on cluster stems are black and sunken, and can girdle parts of the cluster rachis causing the cluster or parts of the cluster to break off or shrivel.

Fig. 8 Numerous lesions concentrated at the base of the oldest internodes result in larger scabby areas that weaken the shoot.

When berries are infected, they can remain symptomless until ripening when they turn brown and become studded with small pimple-like fruiting structures of the fungus (Figure 10) often resembling black rot infected berries.

Fig. 10 Phomopsis fruit rot on ripe Vignoles and Niagara grapes.

However, even though direct fruit infection by both pathogens can occur during the same peak susceptibility period (bloom through 3-4 weeks after bloom), black rot fruit rot symptoms become observable while berries are still green, whereas Phomopsis fruit infections lay dormant until after ripening. Also, leaf symptoms of these two diseases are very different from each other and can be used to determine which pathogen(s) are present and most likely to have caused disease on nearby fruit.

Cultural and chemical control:

Hand pruning to remove dead wood and pruning stubs from the trellis removes much of the over-wintering inoculum of Phomopsis. For this reason, cane pruning can reduce the disease compared to a cordon system that retains a maximum amount of older wood. Trellis systems that train shoots upward also reduce infections on the oldest shoot internodes and clusters. And of course, the maintenance of an open canopy where fruit and other susceptible tissue dry out as quickly as possible after rainfall, will help minimize disease development. For wine grapes, fruit zone leaf removal and shoot thinning reduce canopy density, hasten drying after rainfall, and improve fungicide penetration and coverage of the fruit.

Phomopsis management with fungicides should continue through the first or second post bloom spray, after which inoculum of the fungus is generally spent. Strobilurins, mancozeb products, Captan, and Ziram are generally the only effective materials for Phomopsis control. Some formulations of sterol inhibitor fungicides claim Phomopsis control, but their level of efficacy is still under question and would not be recommended for management of this disease.

This is the first of two posts on grapevine canopy management in the early growing season from bud burst to bloom. The second in the series will be post in two weeks and will focus on pre- or trace-bloom leaf removal for crop level and disease pressure control.

This week, our blog post will focus on shoot thinning, the first canopy management practice of the growing season. As seen in the pictures below, we spent last week shoot thinning Grüner Veltliner (V. vinifera) vines in a central Pennsylvania vineyard (Figure 1).

In the following sections, we will highlight the benefits and costs associated with shoot thinning while providing a few general shoot thinning guidelines for both V. vinifera and hybrid cultivars in the Mid-Atlantic region.

Benefits of Shoot Thinning Grapevines

While dormant pruning (https://psuwineandgrapes.wordpress.com/tag/dormant-pruning/) is the primary tool used by grape growers to maintain vine structure, canopy architecture and regulate crop level, shoot thinning provides an additional canopy management tool to bring vines into vegetative and fruiting balance by reducing shoot density and the number of clusters per vine. Cluster thinning later in the season may be needed in order to balance highly-fruitful vines.

In addition to improving balance between vegetative growth and fruit biomass, other benefits of shoot thinning include:

Reduction of canopy density and fruit shading: through removal of selected shoots, shoot thinning reduces overcrowding of shoots in the canopy thus reducing the number of leaf layers and improving sunlight exposure to fruit (1).

Reduction of disease pressure: reducing canopy density improves air circulation and sunlight penetration that promotes quicker drying of leaves and fruit, as well as increases spray penetration.

Timing of Shoot Thinning

Shoot thinning should be done early in the growing season, when shoots are approximately 5-6 inches long and not more than 10-12 inches long. Shoot thinning should be timed after the date of last ‘expected’ frost, such that secondary or non-damaged primary shoots can be retained in the event of a late spring frost.

When shoot thinning is performed before inflorescences are visible (shoots 0.8 inch to 4 inches), increased vigor of the remaining shoots and lateral shoot growth may occur as a response, negating the benefits of shade reduction (1). When performed too late (shoot longer than 10 inches), shoots become lignified at the base and difficult to remove. If performing late thinning, pruning shears should be used if there is risk of damaging the arm of the vine. It also takes longer to thin longer shoots, potentially decreasing the cost-effectiveness of this practice.

Shoot Spacing and Density Recommendations

Generally, shoot thinning on cane-pruned vines is easier, faster, and more straight-forward than spur-pruned vines, which require substantially more decisions regarding what shoots to retain or remove, and where shoots should be spaced along the cordon (2; Figure 2).

Plant genotype, soil, and climate are all factors influencing vine vigor potential and capacity to fully ripen a crop. Therefore, these factors indirectly affect the appropriate number of shoots to retain at thinning. Many Cooperative Extension websites provide recommendations on range of optimal shoot density based on cultivars grown in their region. [Author’s note: for the eastern US see the additional resources section at the bottom of the post.]

Shoot density targets for Pennsylvania regions:

For vinifera cultivars it is recommended to leave 3 to 5 shoots per linear foot of canopy (3, 4; Figure 3). The general rule of thumb is to retain fewer shoots in red varieties and more in white varieties. However, other factors (i.e., cultivar disease susceptibility) must be taken into consideration.

Figure 3. Suzanne Fleishman, graduate student at Penn State in the Centinari lab, is shoot thinning spur-pruned Grüner Veltliner vines (May 26, 2016). Note the differences shoot density between the cordons on the right (thinned) and on the left (unthinned) cordons.

For most of the hybrid cultivars it is recommended to leave 4 to 6 shoots per linear foot of canopy (5).

For Concord and other native cultivars, as many as 15 shoots per linear foot of canopy can be retained (4).

In divided canopies trellis systems, the same shoot density along each cordon should be retained (Figure 4).

In addition to the number, the position of the shoots along the cordon is important. Ideally, the shoots retained should be equally spaced to promote a uniform, balanced canopy.

The associated costs with manual labor and labor shortages are reasonable considerations before implementing vineyard management practices. This is also true for implementing shoot thinning techniques into a vineyard. Nonetheless, it is also important to consider the potential benefits from implementing a new practice.

The effects of shoot thinning practices on hybrid varieties are a bit unclear. A previous study on shoot thinning found that shoot thinned Marechal Foch (red interspecific hybrid of Vitis) vines exhibited higher total soluble solids (ᵒBrix) and berry anthocyanin concentrations as compared to un-thinned vines (6). The increase in berry anthocyanin, however, did not translate into higher anthocyanin concentration in the final wine, and furthermore, shoot thinning did not impact the sensory perception of “fruitiness” of the wines (6). In contrast, a study focusing on Corot noir (red interspecific hybrid of Vitis) implementation of shoot thinning provided inconsistent results in grape and wine quality across a two-year (2008-2009) evaluation, which was determined by ᵒBrix, pH, titratable acidity (TA), wine anthocyanin, berry and wine tannin content (7). Shoot thinning increased berry ᵒBrix, wine alcohol concentration and anthocyanin content only in second year of this study. While berry TA at harvest was lower (e.g., 2008, un-thinned = 8.6 g/L, shoot thinned = 7.6 g/L), there were no differences in the TA of wine in either year (7). Shoot thinning also decreased berry seed tannin in 2008 and berry skin and wine tannin in 2009, which could have negative implications for final wine, considering generally low tannin concentrations in hybrid red wines (7). In an effort to compensate for costs associated with shoot thinning and yield loss, this study on Corot Noir suggested growers increase the price of grapes by 11 to 20% per ton, depending on the average annual market price and yield loss (7).

A study in Fayetteville (Arkansas) on three highly-fruitful French-American hybrid cultivars (Aurore, Chancellor, and Villard noir) found that shoot thinning increased fruit sugar accumulation (ᵒBrix) only in Chancellor and without changes in pH or TA, while a more intense juice color was associated with shoot thinned vines of both red cultivars (Chancellor and Villard noir; 8). In addition, shoot thinning favorably decreased the Ravaz index (yield to pruning weight ratio) for all three cultivars, improving vine balance (8).

The results of these studies suggest that in some situations the costs of shoot thinning may not outweigh the benefits, especially for hybrids that do not command a high market value (Finger Lakes Grape Prices 2016). However, none of these studies account for potential reduction in disease infections, which may help justify the implementation of shoot thinning in a given vineyard. For example, it has been found that higher shoot density may contribute to the increased incidence of Botrytis rot infections in susceptible cultivars such as Seyval Blanc (9) and Vignoles (4).

In other cases, shoot thinning improved fruit composition in Pinot Noir and Cabernet Franc for two consecutive vintages (1), and also increased color intensity, phenolic content, and total anthocyanins of Cabernet Franc berries (1). Benefits of shoot thinning on fruit quality and wine sensory perception have been reported for other vinifera cultivars, such us Barbera (10) and Sauvginon blanc (11).

Unless your vineyard is located in a low or moderate vigor site, shoot thinning is strongly recommended for vinifera cultivars growing in the Mid-Atlantic region.

If you want to assess the effects of shoot thinning on fruit composition, plan to leave half of a row of vines un-thinned and thin the remaining half to a consistent number of shoots per foot (e.g., 4 shoots per foot). Alternatively, use two rows (of the same variety and cultivar) to assess the impact of shoot thinning in your vineyard: one row thinned and the adjacent row un-thinned. These two methods should help evaluate the effect of shoot thinning on berry composition at harvest and if possible, on wine chemistry and sensory perception assuming that the lots of berries can stay separated through wine production.

Effects of shoot thinning on vine physiology

Impacts of shoot thinning on vine physiology and performance are complex. A study conducted in Italy evaluated the whole-canopy photosynthetic response to shoot thinning using spur-pruned Barbera vines (V.vinifera; 10). Vines were thinned to 5 shoots per foot, reducing the total shoot number by 50% as compared to un-thinned control. In this study (10) shoot thinning significantly improved grape sugar content, color, and phenolics. Despite the benefits provided by shoot thinning on fruit composition, which has been already reported by other studies, what makes this study unique and interesting it that they investigated the mechanisms behind the improvement in grape quality through the measurement of whole-canopy net carbon assimilation. Although the shoot-thinned vines had initially lower photosynthesis (carbon assimilation) than un-thinned vines due to the removal of photosynthetic source (leaf), they had regained photosynthetic capacity to levels similar to the un-thinned vines within 17 days of treatment. This occurred as a result of a substantial increase in both main leaf size and amount of lateral leaves as a result of shoot thinning (10). Therefore, individual shoots of thinned-vines had a higher supply of assimilates (e.g., sugar) per unit of crop, which can increase sugar accumulation during ripening. This may explain why shoot thinning improved grape composition in Barbera under these growing conditions.

Maria Smith is a viticulture PhD candidate with Dr. Michela Centinari in the Department of Plant Science. She specializes in cold stress physiology of wine grapes. She was the previous recipient of the John H. and Timothy R. Crouch Program Support Endowment, an endowment founded and funded by the Crouch brothers, original owners of Allegro Winery in Brogue, PA. She is currently funded by the Northeast Sustainable Agriculture Research and Education (NE-SARE) program, a program from the USDA National Institute of Food and Agriculture (NIFA).

Another season of grape growing is upon us and it’s a good time to review important disease management principles and be aware of some of the tools to consider integrating into your vineyard management programs this spring.

First is your annual reminder to check out the NEWA website (Network for Environment and Weather Applications) found at http://newa.cornell.edu. On the home page is a map of the Northeastern U.S. marked with the locations of hundreds of weather stations where historical and ‘up to the hour’ weather data can be viewed. Although is provided free on the internet, it is funded through the New York State IPM program. Click on a weather station near enough to you (denoted by a leaf/rain drop icon) to get weather, insect pest, and disease information you need to make important management decisions that could save you time and money. Clicking on ‘grapes’ under ‘crop pages’ will give you access to forecasting models for all the major diseases, as well as the grape berry moth degree day model that will improve your timing of grape berry moth insecticide sprays later this summer. Each model forecast is accompanied by helpful disease management messages and explanations.

Next, let’s move our minds into the upcoming pre-bloom disease management season. It’s important to recognize that the threat of disease this spring (pre-bloom) is largely determined by the amount of overwintering inoculum in your blocks. The amount of overwintering inoculum is dependent on the amount of disease that developed in your vineyard last year or in previous years. In other words, if you have kept diseases well under control in the past, especially last year, then there will be relatively little for pathogen populations to build on and cause damage, at least initially, this year. Some very practical research by Wayne Wilcox at Cornell nicely illustrates this point with powdery mildew (pm) development in susceptible wine varieties. In blocks where pm was well controlled all season, fewer overwintering structures of the fungal pathogen (chasmothecia) were available the following spring to jump start disease cycles. Early disease pressure was relatively low and early sprays were less critical to good commercial control than in blocks where disease control was poor the previous year. Where there was poor control the previous year, more of the pathogen overwintered to start disease cycles the following spring and early sprays were critical to maintaining successful commercial control. This is not to say that a bad year of pm will automatically be followed by another bad year. But it certainly tilts the odds in favor of the pathogen, especially if for some reason, you can’t manage the timely application of your early disease control program (stuff happens). It also doesn’t mean you can slack off this year if you had good control last year. Remember, there’s the weather. The weather ALWAYS plays an important role too. A good illustration of this is an experience by an organic grape grower who, in an extremely wet season, developed a serious, economically damaging case of black rot. In conventionally managed vineyards there are several very effective chemistries to control black rot, but in organic production there are no real effective fungicides, and control of this disease in organic vineyards must rely heavily on cultural measures that reduce the pathogen’s overwintering population. Of course, the grower did everything he could to sanitize the trellis of overwintering fruit mummies and bury mummies that had fallen to the ground to reduce overwintering inoculum. But fortunately, the following year was bone dry during the fruit susceptibility period and black rot was not even an issue. Had the previous wet season been followed by another wet one, I’m quite certain, the battle for control of black rot in that organic vineyard would have required ‘the kitchen sink’ to avoid losses. Unfortunately, we have no control over the weather and accurate forecasts, especially long term, are not something to rely on. But, we can (and should) strive to control overwintering inoculum levels every year and the best way to do that is good, practical, season-long disease control.

So, begin to wrap your minds around the campaign ahead. If you had poor disease control in some blocks last season, have you reviewed your spray records where control failed AND where it worked well? Where it failed, did you use the wrong material at a critical time? I’ve had growers discuss their control failures with me only to discover that their timing was fine, but their choice of material did not cover the disease(s) they intended to control. The number of spray materials, what disease each one controls, and how well each one controls each disease, can be bewildering at times…and the list keeps growing and changing. Also, materials that used to be good choices may have become ineffective due to the development of resistance by the pathogens. For example, materials like the strobilurins (Abound, Sovran, Flint, Pristine) are no longer effective at controlling powdery and downy mildew in many parts of the east. In vineyards where this has occurred, using them during the critical fruit protection period (which used to be a great idea!) can now prove disastrous. The sterol inhibitor fungicides (Rally, Elite, Orius, Mettle, Tebusol, Tebustar, Procure, Viticure, etc) are also exhibiting the effects of resistance by the powdery mildew fungus. Though in most cases they still work on powdery to some extent, they are not appropriate for the critical fruit protection period anymore, around and shortly after bloom (products that include the more active difenoconazole are an exception on less susceptible varieties). However, they may be acceptable for maintaining a clean vineyard outside the critical period. Do you have an accurate grasp on that?

Do you have a firm grasp on the critical fruit protection period? The critical period for fruit protection from all diseases generally extends from ‘just before bloom’ to about 4 weeks later. This is the period when you need to be especially vigilant about minimizing spray intervals, using your best materials that cover all the major diseases (Phomopsis, black rot, powdery and downy mildew), focus on good coverage, etc. It is never profitable to try to cut corners during the critical period. However, if you had heavy amounts of black rot in your vineyard the year before, you should assume you have an unhealthy dose of overwintering inoculum in your vineyard this spring, and prevention of leaf lesions in the fruit zone (which would need to be addressed during the first 3-12” of shoot growth, well before the fruit protection period) would also prove to be critical. This goes for other diseases as well (refer back to the previous example with Wayne Wilcox’ powdery mildew experiment). The pre-bloom presence of visible disease in the fruit zone is a big red flag; it means you’ve got potential for serious fruit loss ahead, especially if weather conditions favor the pathogen (wet, warm, humid, calm, cloudy) during the fruit protection period that follows.

Did you record the relative levels of disease that developed in years past for each of your blocks? In order to do this, you need to be able to identify the various diseases and then scout regularly for them. This takes up valuable time but you can streamline your scouting efforts in many ways. Do you know when you would expect to first see each disease? Downy mildew doesn’t become active until about the 5-6 leaf stage. So, you know you can’t expect to see it until about that time or shortly after that. In which blocks are diseases most likely to occur first? Your block or rows next to the woods would be a good place to start, or perhaps your most susceptible variety. Blocks with the most disease last year would be a good place to start. On which parts of the vine do you expect to see diseases appear first? Can recent weather data help you to determine where to look for the disease? For example, if a black rot infection period occurred 2 weeks ago (and you can find this out easily by searching the NEWA website), would you examine the newest growth, the oldest growth, or would you look for lesions on leaves that were currentlyexpanding and most susceptible 2 weeks ago? The answers to these questions can help you streamline your scouting efforts, save time, and improve your expertise.

Do you fully comprehend the susceptibilities of all the varieties you’re growing? You cannot spray premium Vitis vinifera like the hybrids or natives and expect the same results. What are you going to change this year to address disease control breaches in your vinifera? If you had good control last year, are you ready to do it again this year? OR, do you feel lucky and plan to back off until close to bloom to apply your first spray? I always plan for the worst when it comes to the weather and assume it’s going to be wet, cloudy, and warm; ideal for fungal disease epidemics. Consider that here in the east we are growing a highly vulnerable, susceptible host (wine grapes) on the pathogen’s ‘turf’ (the wet, humid eastern U.S.). The good news is that disease control during the pre-bloom period is generally easier (good spray coverage not a problem, low initial disease/inoculum levels, etc.) and cheaper (can use lower fungicide rates, lower spray gallonage, less expensive materials, less time, etc) than in the post bloom period, and a well prepared pre-bloom disease management program will provide extra insurance against problems during bloom and early fruit set, when your fruit ($) is most vulnerable. Now let’s review the common diseases with some of these questions and concepts in mind.

Phomopsis cane and leaf spot is often the first disease problem we face in the pre-bloom period, particularly where trellis systems maintain lots of old and/or dead wood. That’s because old and/or dead wood is where the pathogen overwinters. Therefore, the more old wood you have in your trellis, the more inoculum you can expect to be battling with this spring. Conversely, cane pruned systems have fewer problems with Phomopsis, and cane pruning/minimizing older wood is an important cultural control for this disease. Fortunately, many areas of PA and other parts of the east experienced a relatively dry spring in 2016, helping to minimize new overwintering infections on year-old wood. But, older cordons and especially dead wood and pruning stubs, can carry overwintering inoculum into many subsequent springs. So, if there was little opportunity for new Phomopsis infections to occur last year, you can still be carrying a fair amount of overwintering inoculum in old cordons and pruning stubs.

During early spring rains, Phomopsis spores flush from lesions on wood and are splashed about to invade any new shoot, leaf, and inflorescence they land on…provided the wetting period/temperature combination falls within a minimum range for infection. The basal-most (oldest) internodes of new shoots are the most susceptible to shoot infections simply because they are closest to the inoculum source; wood. In every trial where I have rated shoot infection of Phomopsis, the most severe lesion development was ALWAYS found (on average) on the first (oldest) internode region of the shoot. Lesion development typically got less severe as my rating progressed through internodes 2, 3, 4, and 5. However, once these internodes become fully expanded after the first few weeks in the season, they are no longer susceptible to lesion development. I rarely see Phomopsis lesion development beyond the fifth internode region. That’s why this disease is best dealt with preventatively, very early, during the first few inches of shoot growth. Infections that occur on the first few internodes of new shoots are not only the most likely to occur, but also the most critical; infections of inflorescences (generally on nodes 2-5) can lead to crop loss early (parts of the inflorescence may be ‘bitten off’ by the pathogen) or later during ripening (cluster stem infections in spring move into berries and cause fruit rot and shelling after veraison). And, infections that occur on the basal-most internodes, can’t all be eliminated by judicious hand pruning during the dormant season. So, in blocks where you suspect any risk of early Phomopsis infections, applications of a fungicide (mancozeb or captan are good choices) at no later than 3-6” of shoot growth are a good investment, particularly if you are not cane pruning. Following up with fungicides at 8-12” shoots and immediate pre-bloom are also important pre-bloom applications. Below are some pics from last year’s blog (Figures 1, 2) to help you get a handle on the appearance of lesions on year-old canes. Unfortunately, determining the presence of Phomopsis on older wood generally involves more than just a visual assessment.

Figure 1. Dark brown lesions on the first few internodes on these Chancellor canes are from Phomopsis infections that occurred during early shoot growth in the previous year (when these were green shoots). The buds present are just ready to burst open with new shoot growth that will be very vulnerable to infection during subsequent rain periods.

Figure 2. Although the 1” shoot stage can be vulnerable to damage from this pathogen, the more critical stage is at 3-6” shoots, when more shoot, leaf, and cluster tissue is exposed and is highly susceptible (below). Note the inflorescence in the upper right picture from which Phomopsis has “bitten off” whole branches, dramatically limiting yield potential for that cluster.

Pre-bloom fungicide applications for Powdery mildew are also prudent during early shoot growth for Vitis vinifera cultivars and highly susceptible hybrids, especially in vineyards where control of this disease may have slipped last year (again, because of lots of overwintering inoculum). The primary inoculum for this pathogen generally comes from overwintering structures of the fungus that are lodged within cracks in the bark of cordons and trunks. Spring rain periods of at least 0.1” of precipitation and temperatures of 50 F or more, are the requirements for release of primary inoculum (ascospores) from the overwintering structures. The more mildew that was allowed to develop the year before, the larger the release of spores in early spring, the more primary infections that are likely to occur, and the more critical the need to control the disease early. Sulfur, oils, monopotassium phosphate, and potassium bicarbonate materials can be good choices for mildew management early on. All of these materials can eradicate small existing powdery mildew infections on leaves and cluster stems. Most do not generally offer any protection from future infections and therefore work best if applied often. Sulfur is an exception, and has the added benefit of providing a week or more of protection against future infections. Many of the more experienced growers like to utilize a mancozeb/sulfur combination to control all diseases during the pre-bloom period. This combination is relatively inexpensive, there are no resistance issues, and it works. Remember to read labels and be aware that you can’t mix sulfur and oils, or oils and captan. The tebuconazole products can be used during early pre-bloom to control powdery mildew as well, especially at the 8-10” shoot stage. These materials are very inexpensive and generally provide enough powdery mildew control to keep vines healthy until the immediate pre-bloom spray (they will also nicely control early black rot infections). At immediate pre-bloom and first post bloom, you want to apply your best powdery mildew chemistries like quinoxyfen (Quintec), difenoconazole (Revus Top), metrafenone (Vivando), fluopyram/tebuconazol (Luna Experience), etc. For native juice grapes, powdery mildew is rarely a concern during the early shoot growth stages, especially in the cooler Lake Erie region of Pennsylvania.

A note about fungicide resistance management and powdery mildew: It’s important to plan your powdery mildew management choices ahead of time with resistance management in mind. The easiest way to do this is to become familiar with FRAC (fungicide resistance action committee) codes listed prominently on the first page of fungicide labels. Fungicides with the same FRAC group number can be considered similar enough in their mode of action/chemistry that resistance to one is resistance to all others within that group. Therefore when you rotate fungicides for resistance management, you’re essentially rotating FRAC groups. Some good rules to remember are to avoid using the same FRAC group consecutively, or more than twice in a given season. The development of powdery mildew resistance is always a concern when using materials like the strobilurins (FRAC 11), the sterol inhibitors (FRAC 3), Quintec (FRAC 13), Vivando (FRAC U8), Luna Experience (FRAC 7, 3), Torino (FRAC U6), and Endura (FRAC 7) to name a few. Resistance is generally not a concern for uses of sulfur, oils, bicarbonates, and the potassium salts (mentioned above), or copper.

Next, black rot: One of the best ways to reduce overwintering inoculum of black rot is to scout your vineyard for old fruit mummies and eliminate them from the trellis. Black rot infected fruit mummies that have overwintered in the trellis are the most potent source of inoculum for infections the following spring. No matter how cold it gets over the winter, the pathogen survives just beautifully in colonized fruit remaining in the trellis. But, dropping this inoculum source to the soil, allows microbial degradation/weathering to reduce the potential for mummies to release spores the following spring. It also places the inoculum source much farther from new, susceptible plant tissue up in the trellis. The best time to ‘sanitize’ the trellis is during dormant pruning; weathering has already accomplished some of the removal of last season’s infected fruit from the trellis, and what remains is relatively easy to see and remove by hand. Experiments we conducted several years ago clearly showed that the earlier the mummies are knocked to the ground during the dormant period, the more time for decomposition to break them down before the next season, and the fewer spores released from the ground the following spring to start new disease cycles. Nevertheless, some inoculum on the ground will survive to release spores in spring, and burial of mummies with cultivation will go a step further to eliminate the threat. Removal of ALL old cluster material from the trellis before bud break is important to maintaining good control of this disease.

It may not be necessary to apply a fungicide for black rot at early shoot stages IF good control of this disease was achieved the previous year AND conscientious scouting and trellis sanitation has been implemented. However, the importance of early shoot infections should not be underestimated as I mentioned above, especially if they result in leaf lesions in the fruit zone. For example, inoculations we performed from early May to early June (simulating wet weather and an overwintering inoculum source (mummies) in the trellis) resulted in leaf and shoot lesions in the cluster zone (Figure 3). Those lesions went on to release spores during the critical fruit protection period, resulting in crop loss of 47-77% on those shoots with infected leaves!

An application of mancozeb, ziram, or captan for Phomopsis will also provide control of early black rot infections. The sterol inhibitor fungicides and strobilurins are also good materials for black rot that are more rainfast than mancozeb, ziram, and captan. The sterol inhibitors also provide excellent post infection activity that can be very useful at terminating an infection that has already occurred (but not yet manifested itself).

Figure 3. Early (pre-bloom) black rot leaf infections in the cluster zone provide inoculum that can add to problems with controlling fruit infection after capfall. The two small tan lesions on the leaf at node 2 are just inches from the developing inflorescence found at node 3 (picture on the right). These lesions will release spores during rainfall periods that could easily be splashed to highly susceptible cluster stems pre-bloom, and developing fruit after capfall. Resulting fruit infections will lead to crop loss.

Downy mildew and the 5-6 leaf stage: This stage marks the point at which the downy mildew pathogen first becomes active and is capable of releasing primary spores from inoculum sources that have overwintered on the ground (leaves and other plant material that was infected during the previous season). As with all other diseases, vineyards that developed a fair amount of downy mildew leaf/cluster infection last year will be at higher risk this spring than vineyards that were kept clean. However, overwintering structures of the downy mildew pathogen can survive more than one season in the soil.

Periods of rainfall with temperatures of at least 52 F meet the requirements of spore release and the first infections; plant surfaces must be wet for infection to occur. While scouting for this disease, expect to see it first in wetter areas of your acreage and pay close attention to leaves near the ground (sucker growth, grape seedlings that germinated from shelled berries last fall) which are most likely to become infected first. Therefore, keeping such low growth to a minimum in spring is a prudent control measure that can delay the development of the disease. It also suggests that if you’re planning vine trunk renewal from sucker growth, you will need to apply fungicides to protect that growth from the ground up as the pathogen becomes active.

Spring leaf infections are identified by the yellow ‘oil-spots’ seen on the tops of leaves (Figure 4), coinciding with white, downy sporulation of the pathogen on the undersides of leaves. Inflorescences can be blighted and show sporulation as well. Sporulation occurs during darkness under high relative humidity, and can typically be seen during a morning scout of the vineyard following a wet/humid night. Under optimum temperatures (70-75F), only an hour or two of plant surface wetness may be required for infection to occur, and new infections can produce their own spores with just 5 days.

Many parts of the northeast experienced drought conditions last year, which severely inhibited the development of this disease. Up in Erie County PA, the disease basically took a vacation in 2016, and I could barely find a handful of lesions on unsprayed ‘Chancellor’ leaves and fruit near the ground all summer: it was the perfect year to start renewal trunks! It wasn’t until later in August that rains finally returned and we began to see a few more infections, but for the most part the disease literally could not get off the ground in Erie county PA in 2016. What does this mean for 2017? The great lack of downy mildew in drought hit areas last year means that pre-bloom disease cycles this year will have to rely on overwintering inoculum from previous years (although spores of downy mildew can travel long distances between vineyards, the first infections will arise from inoculum within your vineyard). I have not found any detailed information as to how long the pathogen can survive in the soil, but I guarantee that if you’ve had downy mildew before, then it’s still there. Whether your area was wet or dry last spring, the principle described earlier still applies: vineyards devoid of downy mildew last year (whether from drought or just plain good control) will be easier to keep ‘clean’ in the pre-bloom period this year.

Mancozeb products are good options for the first downy mildew, Phomopsis, and black rot sprays in the pre-bloom period. Ziram and Captan have a similar spectrum of control, but Ziram is a little weaker on downy mildew, and Captan a little weak on black rot. However, these may be a viable option if these diseases are not a huge threat early on (that is if you had good control last year). These materials are all surface protectants subject to wash-off by rainfall, which means that under heavy, frequent rainfall conditions, application intervals will need to be minimized (7-10 days?) especially for highly susceptible varieties. For that more critical ‘immediate pre-bloom’ spray (and the first post bloom spray), there are other materials like Presidio, Revus, Revus Top, and Zampro that are quite rainfast, very effective, and will provide longer range protection under wet conditions (when you need the protection most and are least likely to be able to stick to shorter spray intervals). However, products like Presidio also require a second active ingredient (like mancozeb) in a tank mix for resistance management purposes (which isn’t a bad idea at this critical spray timing in any case). Other materials like the phosphonates, Ranman, and the strobies /Reason, are probably best utilized outside the critical two sprays around bloom (especially for V. vinifera and highly susceptible hybrids), unless they’re used as tank mix partners with other effective materials. They’re very good materials, but they’re just not the ‘best of the best’.

One more time for emphasis: the immediate pre bloom and first post bloom (7-14 days later) fungicide applications are the most important you’ll make all year, regardless of variety grown and disease pressure. These two sprays protect your fruit from all the major fungal diseases (Phomopsis, black rot, downy and powdery mildew). Make sure sprayers are properly calibrated and adjusted for best coverage on a bloom-period canopy, spray every row at full rates and shortest intervals, and NEVER extend the interval between these sprays beyond 14 days.

‘Newer’ Fungicides: Aprovia (solatenol) may be worth a try for powdery mildew control (received federal registration in 2015). The active ingredient is related (same FRAC group) to Boscalid (found in Endura and Pristine) and Fluopyram (found in Luna Experience). It also has activity against black rot, but should not be expected to control this disease under high pressure on a susceptible variety.

***Lastly, to help you with all your grape management decisions this year, you should have…

We are starting to see increases in herbicide management costs.[1] Some of you know all to well that 1-2 applications of herbicide do not provide adequate control of weed competition in vineyards. Complicated tank mixes that cost over $100 per applied acre are not a practice I would consider sustainable. Some growers, though, would disagree.

The cost of materials are not increasing substantially. More frequent applications and a need to apply better materials more often is driving costs up. The majority of herbicides used by growers are off patent these days and available almost exclusively in generic form. A third or even fourth vineyard pass, could be sustainable. The cost of materials and materials selected needs to be looked at comprehensively with the number of passes required to obtain adequate control.

There may be a potential for cover crops to improve the effectiveness of weed control.[2] We can observe this not just in row middle management, but to a lesser extent under trellis management. Cover crops do not offer the potential to reduce herbicide applications in situations where growers are applying between 1 and 3 per year. Rather, they offer an option to improve results without adding an additional pass. This is because cover crops can reduce vine size when row middle competition is undesirable. In 2016 field trials we observed smaller berry size when cover crops were planted in the late summer of 2015 and were not terminated before June 1, 2016. Particularly where hard to control species get established, some growers have added a late summer or fall application to bring their total number of herbicide application to 4-5. In this scenario, the right cover crop mix offers the potential of superior control with one less pass. Cover crops do require some form of termination (usually chemical). By selecting the right species, a low rate of round-up may offer excellent row middle control.

Figure 1: Side by side cover crop trial in a commercial vineyard showing cover crop suppression of Horseweed (Marestail) pressure. Image B is the control and shows significant Horseweed (Marestail) pressure. Photos by Luke Haggerty, LERGP

Figure 2: Cover crops are under the vines on the left side of the image, while the under vine (or under trellis) area of the vines on the right is managed with herbicide. Photo by Suzanne Fleishman, a graduate student that works with Dr. Michela Centinari.

Cover crop mixes being trialed are similar in cost to an herbicide application. Low-end rye grass and radish blends are comparable to many post emergent row middle applications. Higher end seed mixes with oats, more radishes or even buckwheat range between $12 and $15 per seeded acre in materials. Legumes increase costs but potentially reduce fertilizer use.[3] Easy to kill hybrid crimson clover complicates the economic analysis. It may reduce urea applications by 50%, but could be more difficult to grow. Understanding effective seed mixes, their primary benefits and potential secondary benefits will be key to the success of moving cover crops into perennially systems in a cost-effective (saving) way. Regional differences in seed prices also complicated the matter. One of our primary suppliers of cover crop seeds in the Lake Erie Region is Ernst Seed Co. 2016 prices were used to calculate the cost of various seed mixes used in trials.

LERGP, led by Luke Haggerty, is taking an integrated look at cover crops in Concord vineyards. As he observes benefits, I’ll help quantify them. There is a lot we still do not know. While preliminary results show promise for increasing economic sustainability where herbicide program prices are spiraling upward, a few years’ worth of data will allow us to clearly observe measurable benefits in herbicide programs. Right now, it just seems like there are less weeds and more cover crop in row middles that have been seeded.

Another harvest will soon be over for grape growers in Pennsylvania and the winter season is fast approaching. Take the time this winter to explore the resources below to prepare for next season’s pest problems.

Hardcopy References
The following 5 references provide information on identification and management of insect, disease and weed problems in vineyards. I suggest purchasing these items before next season begins. Although the cost will be over $250 it is well worth having these invaluable resources in your viticultural library.

New York and Pennsylvania Pest Management Guidelines for Grapes: Every commercial grape grower in Pennsylvania should have a copy of the current guidelines. This guideline provides a wealth of information on insect, disease and weed management with pesticide options, rates, and schedules, as well as, a chapter on sprayer technology.

Compendium of Grape Diseases, Disorders, and Pests, Second Edition: This new edition is an expanded version of the original Compendium with 375 photos and drawings and containing updated information about pathogens including additional diseases. The second edition is divided into 4 parts covering: diseases caused by biotic factors (e.g., fungi, bacteria, viruses etc.); disease – like symptoms caused by insects and mites; disorders caused by abiotic factors (e.g., environmental stresses, nutritional disorders, etc.); and fungicides/spray technology.

Weeds of the Northeast: Described as the first comprehensive weed identification manual available for the Northeast enabling identification of almost 300 common and economically important weeds in the region. The manual contains color photos of vegetative and flowering stages of weeds, as well as, seed photos.

Wine Grape Production Guide for Eastern North America: A comprehensive reference on all aspects of wine grape production (e.g., varieties, canopy management, nutrient management, etc.) including chapters on disease management, insect and mite pests and vineyard weed management.

Important viticulture resources for vineyard managers in the Mid-Atlantic region.

Grape Insect and Mite Pests – 2016 Field Season by Greg Loeb, Cornell University (21 pages). Dr. Loeb provides a thorough review of insect pests that you might see throughout the season in the vineyard. Included are 18 photos of pests/injury along with management guidelines.

NYS IPM : Fruit IPM Fact Sheets (Cornell): Fact sheets on diseases and insects on grapes, tree fruit and small fruit. A total of 22 fact sheets pertain to insects and diseases on grapes.

Identifying Grape Insects (Michigan State University): The information on this site is from the previously mentioned resource, A Pocket Guide for Grape IPM Scouting of Grapes in North Central & Eastern U.S. and is categorized by: Pests attacking; buds, leaves, fruit, root, during harvest. Also includes beneficial insects and mites.

Growing Grapes – Vineyard IPM (eXtension): Articles both in English and Spanish on: insects, diseases, weeds, animal pests and problems not caused by insects or diseases.

Weed Resources – Web sitesNew Jersey Weed Gallery (Rutgers): Photos and descriptions of weeds found in New Jersey. Weeds can be viewed by common name, Latin name or thumbnail images.

Weed Identification Guide (Virginia Tech): These pages are intended to aide in the identification of common weeds and weed seedlings found throughout Virginia and the Southeastern U.S. The weed pictures are arranged alphabetically by common name.